The invention relates to glass beads for use in optical applications, devices made from such glass beads, and a method of screening glasses for use in optical devices.
Glass beads and microspheres are known in the art and have been developed for and used in a variety of applications. Glass beads have been used in retroreflective products, as fillers, in propping and peening applications, and in optical devices. Compositions of these known beads have generally been limited to traditional glass-forming compositions, or to high refractive index compositions with favorable melting and processing behavior. For example, pavement marking beads of soda-lime-silica glass comprising about 70 percent silica are common. High refractive index beads typically comprise less silica and have substantial amounts of titania, baria, lead, or bismuth. High index pavement marking beads have been doped with rare earth elements to provide visibility-enhancing fluorescence. Beads for mechanical uses often have significant amounts of alumina or zirconia.
Beads used in optical devices have been derived from high purity optical materials such as optical fibers and laser glass. Such materials have provided the desired ultra high-Q factors, or low losses, desired for resonator devices. Accordingly, beads of this type have included pure silica beads, pure fluoride glass beads (so-called ZBLA and ZBLAN), and beads of high phosphate laser glass. For resonator applications, these glasses are sometimes doped with a low level of a rare earth agent to make them optically active. Beads made by melting the tip of an optical fiber comprise primarily pure silica or pure fluoride glass, but have a very small center region comprising additional components derived from the core region of the optical fiber.
In addition, high purity glass is essential for applications in which the glass transmits light, as in a waveguide, optical fiber, or in high-Q resonators. It is known that transition metal or rare earth impurities can strongly absorb visible and infrared light, which leads to increased optical loss in a device. For example, transition metals, such as iron, copper, and vanadium, have crystal field splitting energies in the 1-10 eV range (xcx9c1240-xcx9c125 nm) and broad absorption bands, which are deleterious in the visible and near-IR regions. The presence of iron (III) in silica, for example, can lead to an induced absorption of 130 dB/km/ppm at 1.3 xcexcm. Similarly, rare earth ions exhibit strong, but narrow, absorption bands in the visible and IR spectra. For example, Tb3+ in fluorozirconate induces a 150 dB/km/ppm absorption at 3.0 xcexcm in fluorozirconate.
In addition, impurity ions can alter the local structure of a glass and lead to different crystal field environments around nearby cations. In the case of rare-earth-doped glasses, the local field dictates the lifetime and breadth of the emission spectrum. As the use of high purity glass is essential in transmission applications, for example, amplifier optical fiber, it is prudent to use high purity glasses to screen compositions in order get the most accurate information about how that glass will perform in an optical device.
In silicates, hydroxyl ions impart unwanted absorption bands at 2.75, 2.22, 1.38, 1.24, and 0.95 xcexcm. The 1.38 xcexcm absorption band is particularly problematic for telecommunications applications. In silicate optical fiber, the absorption results in about a 40 dB/km/ppm hydroxyl ion loss at 1.38 xcexcm. Thus, it is desirable for telecommunications devices and waveguides to have hydroxyl concentrations less than about 1 ppm. The presence of hydroxyl ions has also been reported to decrease the excited-state lifetime of rare-earth-doped glasses. Hydroxyl ions also modify the viscosity of the glass. The log viscosity decreases about 0.0018/ppm hydroxyl ion. For example, 100 ppm hydroxyl ions in the glass decreases the viscosity of the glass by approximately 40 percent.
What is needed are homogeneous beads, processes of making, and devices comprising high purity, tailored compositions that provide non-maximal Q-factors. Such beads would be useful for broadening selected frequency bands in resonators, for photo-trimmable devices, and for screening glass compositions for making optical devices, for example, optical fibers.
In one aspect, the invention provides an optically active solid glass bead comprising greater than 80 weight percent silica, one or more active rare earth dopants, and one or more modifying dopants. The modifying dopant may be either a cationic or anionic species.
In another aspect, the invention provides a solid glass bead comprising a mixture of greater than 80 weight percent silica and at least two active rare earth dopants.
In another aspect, the invention provides a photosensitive solid glass bead that comprises greater than 80 weight percent silica and germania combined. Germania is present in the solid glass bead in an amount of at least 5 weight percent. The glass beads comprising silica and germania are photosensitive. The solid glass beads comprising silica and germania may also further comprise one or more modifying dopants.
In another aspect, the invention provides a solid glass bead that comprises from about 20 to about 90 anion mole percent of at least one non-oxide anion.
Embodiments of the glass beads of the invention may typically contain about 100 ppm of hydroxyl groups or less, or less than about 1 ppm of hydroxyl group.
The glass beads of the invention may contain a variety of levels of active rare earth and modifying dopants and contain an effective amount so to provide an optically active glass bead. The compositions of the glass beads of the invention are preferably functionally homogeneous.
In another embodiment, the invention provides a method of making solid glass beads comprising the steps of forming a solution comprising glass precursors; converting the precursors to glass precursor powder; heating the glass precursor powder in the presence of halogen gas to dehydrate the glass precursor powder; and exposing the glass precursor powder to a flame to form solid glass beads.
In another aspect, the invention provides a method of making an optical device comprising a glass composition by using solid glass beads to screen glass compositions. In one embodiment, the method of the invention comprises the steps of: providing at least one solid optically active glass bead of each of at least two glass compositions to be screened for desirable properties; exposing at least one glass bead of each of the at least two glass compositions to light; collecting emitted light from at least one glass bead of the at least two glass compositions; analyzing the emitted light; selecting a glass composition having the desired properties; and incorporating the selected glass composition into the optical device, wherein said glass composition is in a form other than a solid glass bead. The above method of the invention is not limited to the glass beads of the invention and includes any optically active glass or glass-ceramic beads.
In another aspect, the invention provides a method of altering the refractive index of a photosensitive glass bead comprising the step of exposing the glass bead to actinic radiation.
In another aspect, the invention provides a method of altering the output of an optical device comprising a glass bead comprising the step of exposing the glass bead in the optical device to actinic radiation wherein the output of the device is altered.
In another aspect, the invention provides optical devices comprising solid glass beads of the invention.
xe2x80x9cActive rare earth dopantxe2x80x9d means a rare earth dopant that provides light emission in response to excitation of its electrons.
xe2x80x9cOptically active glassxe2x80x9d generally means a glass that provides: a useful response to, modulation of, or manipulation of, incident light. Specific examples of optically active glass include, but are not limited to, glass which exhibits: fluorescence, stimulated emission, birefringence, photosensitivity, and optical nonlinearity.
xe2x80x9cBirefringencexe2x80x9d means different refractive indexes along different directions.
xe2x80x9cPhotosensitivityxe2x80x9d means a change in refractive index of at least 1xc3x9710xe2x88x925 from exposure to light.
xe2x80x9cOptical nonlinearityxe2x80x9d means that the refractive index or the absorption coefficient is a function of the intensity of light.
xe2x80x9cFunctionally homogeneousxe2x80x9d means materials that provide continuous regions of the selected compositions assessable for light propagation that are exclusive of inhomogeneous features or of regions having differing compositions. xe2x80x9cDiffering compositionsxe2x80x9d means the dopant ion concentration in any 1 micrometer cross section within the regions of the selected compositions does not vary by more than 20 percent as compared to the regions of selected composition outside the 1 micrometer region.
xe2x80x9cLightxe2x80x9d means electromagnetic radiation of any wavelength and includes, for example, UV, visible, infrared, x-ray, microwave, radiowave, and gamma ray.